In a large bowl, whisk together 3 tablespoons oil, vinegar, and Dijon; season with salt and pepper. Set aside.
In a large nonstick skillet, heat remaining tablespoon oil over medium. Sprinkle chicken with coriander; season with salt and pepper. Cook (this may need to be done in batches), turning once, until opaque throughout, 4 to 5 minutes. Transfer to a cutting board and let rest for 5 minutes; slice thinly crosswise.
Add chicken, spinach, grapes, shallot, almonds, and corn to bowl with dressing; toss to combine. Top with avocado. Serve immediately.